Iptables is a firewall that plays an essential role in network security for most Linux systems. While many iptables tutorials will teach you how to create firewall rules to secure your server, this one will focus on a different aspect of firewall management: listing and deleting rules.

In this tutorial, we will cover how to do the following iptables tasks:

List rules
Clear Packet and Byte Counters
Delete rules
Flush chains (delete all rules in a chain)
Flush all chains and tables, delete all chains, and accept all traffic
Note: When working with firewalls, take care not to lock yourself out of your own server by blocking SSH traffic (port 22, by default). If you lose access due to your firewall settings, you may need to connect to it via the console to fix your access. Once you are connected via the console, you can change your firewall rules to allow SSH access (or allow all traffic). If your saved firewall rules allow SSH access, another method is to reboot your server.

Prerequisites
Before you start using this tutorial, you should have a separate, non-root superuser account—a user with sudo privileges—set up on your server. If you need to set this up, follow the appropriate guide:

Initial Server Setup with Ubuntu 14.04
Initial Server Setup with CentOS 6
Let’s look at how to list rules first. There are two different ways to view your active iptables rules: in a table or as a list of rule specifications. Both methods provide roughly the same information in different formats.

List Rules by Specification
To list out all of the active iptables rules by specification, run the iptables command with the -S option:

As you can see, the output looks just like the commands that were used to create them, without the preceding iptables command. This will also look similar to the iptables rules configuration files, if you’ve ever used iptables-persistent or iptables save.

List Specific Chain

If you want to limit the output to a specific chain (INPUT, OUTPUT, TCP, etc.), you can specify the chain name directly after the -S option. For example, to show all of the rule specifications in the TCP chain, you would run this command:

target: If a packet matches the rule, the target specifies what should be done with it. For example, a packet can be accepted, dropped, logged, or sent to another chain to be compared against more rules
prot: The protocol, such as tcp, udp, icmp, or all
opt: Rarely used, this column indicates IP options
source: The source IP address or subnet of the traffic, or anywhere
destination: The destination IP address or subnet of the traffic, or anywhere
The last column, which is not labeled, indicates the options of a rule. That is, any part of the rule that isn’t indicated by the previous columns. This could be anything from source and destination ports, to the connection state of the packet.

Show Packet Counts and Aggregate Size

When listing iptables rules, it is also possible to show the number of packets, and the aggregate size of the packets in bytes, that matched each particular rule. This is often useful when trying to get a rough idea of which rules are matching against packets. To do so, simply use the -L and -v option together.

Now that you know how to list the active firewall rules in a variety of ways, let’s look at how you can reset the packet and byte counters.

Reset Packet Counts and Aggregate Size
If you want to clear, or zero, the packet and byte counters for your rules, use the -Z option. They also reset if a reboot occurs. This is useful if you want to see if your server is receiving new traffic that matches your existing rules.

To clear the counters for all chains and rules, use the -Z option by itself:

sudo iptables -Z
To clear the counters for all rules in a specific chain, use the -Z option and specify the chain. For example, to clear the INPUT chain counters run this command:

sudo iptables -Z INPUT
If you want to clear the counters for a specific rule, specify the chain name and the rule number. For example, to zero the counters for the 1st rule in the INPUT chain, run this:

sudo iptables -Z INPUT 1
Now that you know how to reset the iptables packet and byte counters, let’s look at the two methods that can be used to delete them.

Delete Rule by Specification
One of the ways to delete iptables rules is by rule specification. To do so, you can run the iptables command with the -D option followed by the rule specification. If you want to delete rules using this method, you can use the output of the rules list, iptables -S, for some help.

For example, if you want to delete the rule that drops invalid incoming packets (-A INPUT -m conntrack –ctstate INVALID -j DROP), you could run this command:

sudo iptables -D INPUT -m conntrack –ctstate INVALID -j DROP
Note that the -A option, which is used to indicate the rule position at creation time, should be excluded here.

Delete Rule by Chain and Number
The other way to delete iptables rules is by its chain and line number. To determine a rule’s line number, list the rules in the table format and add the –line-numbers option:

Once you know which rule you want to delete, note the chain and line number of the rule. Then run the iptables -D command followed by the chain and rule number.

For example, if we want to delete the input rule that drops invalid packets, we can see that it’s rule 3 of the INPUT chain. So we should run this command:

sudo iptables -D INPUT 3
Now that you know how to delete individual firewall rules, let’s go over how you can flush chains of rules.

Flush Chains
Iptables offers a way to delete all rules in a chain, or flush a chain. This section will cover the variety of ways to do this.

Note: Be careful to not lock yourself out of your server, via SSH, by flushing a chain with a default policy of drop or deny. If you do, you may need to connect to it via the console to fix your access.

Flush a Single Chain

To flush a specific chain, which will delete all of the rules in the chain, you may use the -F, or the equivalent –flush, option and the name of the chain to flush.

For example, to delete all of the rules in the INPUT chain, run this command:

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sudo iptables-FINPUT

Flush All Chains

To flush all chains, which will delete all of the firewall rules, you may use the -F, or the equivalent –flush, option by itself:

sudo iptables -F
Flush All Rules, Delete All Chains, and Accept All
This section will show you how to flush all of your firewall rules, tables, and chains, and allow all network traffic.

Note: This will effectively disable your firewall. You should only follow this section if you want to start over the configuration of your firewall.

First, set the default policies for each of the built-in chains to ACCEPT. The main reason to do this is to ensure that you won’t be locked out from your server via SSH:

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sudo iptables-PINPUT ACCEPT

sudo iptables-PFORWARD ACCEPT

sudo iptables-POUTPUT ACCEPT

Then flush the nat and mangle tables, flush all chains (-F), and delete all non-default chains (-X):

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sudo iptables-tnat-F

sudo iptables-tmangle-F

sudo iptables-F

sudo iptables-X

Your firewall will now allow all network traffic. If you list your rules now, you will will see there are none, and only the three default chains (INPUT, FORWARD, and OUTPUT) remain.

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IPtables isastateful firewall

*You can understand what the below commands doing.

iptables-IINPUT-ptcp-mtcp-s1.1.1.1--dport22-jACCEPT

iptables-IINPUT-ptcp-mtcp-s1.1.1.1/29--dport22-jACCEPT

iptables-IINPUT-ptcp-mtcp-s0.0.0.0/0--dport22-jDROP

*Please save the configuration what you did tothe/etc/sysconfig/iptables.

You could use the UUID of the partition instead of /dev/md126 The UUID of a filesystem is unlikely to change unless you alter it, whereas the partition may change its identifier and be named md0 or md255 or something like that.

Use the blkid or lbslk command to get the UUID, and then replace /dev/md126 with UUID=xxx-xxxxx-xxxxxx-xxx in the above line: